ABC News Chief Health and Medical Editor Dr. Richard Besser says Zika may have been picked up by mosquitos from infected soccer fans and spread across Brazil and the rest of South America.

“One of the theories about how the Zika virus came to Brazil is with the World Cup. People came from all over and someone came in had Zika virus,” he said. “You take one infected person who is bitten by a mosquito. That mosquito can then go and infect other people.”

Now the epidemic is putting this summer’s upcoming Olympic games in Rio in jeopardy. After 500,000 visitors attended the London Olympics in 2012, a similar amount are expected in Rio.

But hundreds of thousands may cancel as the exploding Zika epidemic gets worse.

Yahoo Travel Editor Laura Begley Bloom told INSIDE EDITION: “This virus is not great for upcoming events like Carnivale and the Olympics. People are definitely going to be canceling their travel plans, especially if they're pregnant. I must say I would not want to go to any of these regions, the risks are really high.”

There have been 1 million cases in Brazil since the 2014 World Cup and 4,000 women have given birth to children with heartbreaking defects that include dramatically small heads.